Field of the Invention
The present invention is broadly concerned with novel temporary wafer bonding methods utilizing dual layer bonding systems. The inventive methods can support a device wafer on a carrier substrate during wafer thinning and other backside processing.
Description of the Prior Art
Temporary wafer bonding (TWB) normally refers to a process for attaching a device wafer or microelectronic substrate to a carrier wafer or substrate by means of a polymeric bonding material. After bonding, the device wafer is thinned, typically to less than 50 μm, and then processed to create through-silicon vias (TSV), redistribution layers, bond pads, and other circuit features on its backside. The carrier wafer supports the fragile device wafer during the backside processing, which can entail repeated cycling between ambient and high temperatures (>250° C.), mechanical shocks from wafer handling and transfer steps, and strong mechanical forces, such as those imposed during wafer back-grinding processes used to thin the device wafer. When all of this processing has been completed, the device wafer is usually attached to a film frame and then separated, or debonded, from the carrier wafer and cleaned before further operations take place.
Most TWB processes use either one or two layers between the device wafer and the carrier wafer. In the case of a two-layer system, the first layer is a polymeric bonding material. The polymeric bonding material layer is typically 10-120 μm thick and, more commonly, about 50-100 μm thick. The second layer is comparatively thin, typically less than 2 μm, and is present to enable facile separation of the bonded wafer pair after processing. The second layer, which may or may not be polymeric in nature, usually functions in one of two ways. In the first instance, the second layer creates a non-bonding or weakly bonding interface with the device or carrier wafer surface. This allows the bonded wafer pair to be separated by applying low mechanical force to delaminate the structure at the weak interface. In the second instance, the thin layer responds to radiation from a laser or other light source, which leads to decomposition of the layer itself or decomposition of the adjacent polymeric bonding material, causing bonding integrity to be lost within the structure and allowing it to come apart without applying mechanical force.
Laser-induced release is becoming a popular mode of debonding, and materials are available for operating at laser wavelengths ranging from the ultraviolet (e.g., 248 nm and 308 nm) to the near infrared (e.g., 1064 nm). In some cases, the polymeric bonding material has sufficient response to the laser radiation that a separate, thin light-sensitive layer is not required. Polyimide bonding materials, for example, have very high optical density in the ultraviolet and will readily ablate when scanned with a 308-nm excimer laser at a dosage of about 250 mJ/cm2.
In some processes, two or more polymeric bonding materials have been used to form the bond line in place of a single, polymeric bonding material. In one such process, a first rigid thermoplastic bonding layer that does not flow appreciably at normal wafer bonding and backside processing temperatures (200°-280° C.) can be applied to a device wafer to enhance its mechanical stability while the bonded structure is being processed. The device wafer coated with this first bonding material is then bonded to a carrier wafer that has been coated with a second bonding material, or, alternatively, the second bonding material is coated on top of the first bonding material, and the resulting structure is bonded to a carrier wafer. The second bonding material softens and flows during the bonding process and has the proper surface characteristics to form a strong, continuous adhesive bond with the first rigid bonding material, which is a requirement for the multiple layer bonded structures.
However, in prior art multi-layer structures such as these, the occurrence of strong bonding between the first and second bonding layers creates practical difficulties, especially for cleaning, since both layers will reside on the device wafer after the carrier has been separated from the structure. Because the first and second bonding materials are different compositions, it may be necessary to use a different cleaning solution for each material, and these solutions may not be compatible in the same wafer cleaning system. Furthermore, depending upon the bonding material, wet chemical removal of the two bonding layers may not even be possible. While one can resort to peeling to remove the bonding material, peeling processes (which are analogous to removing an adhesive tape from a surface) are generally viewed as undesirable since they impose large mechanical stresses on the device wafer and tend to leave behind residue. There is a need for new TWB methods that allow for easier separation of the bonded wafers without undue stress or strain on the device features.